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Earth and Planetary Science Letters 260 (2007) 482 – 494
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Intracratonic asthenosphere upwelling and lithosphere rejuvenation
beneath the Hoggar swell (Algeria): Evidence from HIMU
metasomatised lherzolite mantle xenoliths
L. Beccaluva a,⁎, A. Azzouni-Sekkal b , A. Benhallou c , G. Bianchini a , R.M. Ellam d ,
M. Marzola a , F. Siena a , F.M. Stuart d
a
b
Dipartimento di Scienze della Terra, Università di Ferrara, Italy
Faculté des Sciences de la Terre, Géographie et Aménagement du Territoire, Université des Sciences et Technologie Houari Boumédienne,
Alger, Algeria
c
CRAAG (Centre de Recherche en Astronomie, Astrophysique et Géophysique), Alger, Algeria
d
Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, East Kilbride, UK
Received 7 March 2007; received in revised form 23 May 2007; accepted 24 May 2007
Available online 2 June 2007
Editor: R.W. Carlson
Abstract
The mantle xenoliths included in Quaternary alkaline volcanics from the Manzaz-district (Central Hoggar) are proto-granular,
anhydrous spinel lherzolites. Major and trace element analyses on bulk rocks and constituent mineral phases show that the primary
compositions are widely overprinted by metasomatic processes. Trace element modelling of the metasomatised clinopyroxenes
allows the inference that the metasomatic agents that enriched the lithospheric mantle were highly alkaline carbonate-rich melts
such as nephelinites/melilitites (or as extreme silico-carbonatites). These metasomatic agents were characterized by a clear HIMU
Sr–Nd–Pb isotopic signature, whereas there is no evidence of EM1 components recorded by the Hoggar Oligocene tholeiitic
basalts. This can be interpreted as being due to replacement of the older cratonic lithospheric mantle, from which tholeiites
generated, by asthenospheric upwelling dominated by the presence of an HIMU signature. Accordingly, this rejuvenated
lithosphere (accreted asthenosphere without any EM influence), may represent an appropriate mantle section from which deep
alkaline basic melts could have been generated and shallower mantle xenoliths sampled, respectively. The available data on
lherzolite xenoliths and alkaline lavas (including He isotopes, Ra b 9) indicate that there is no requirement for a deep plume
anchored in the lower mantle, and that sources in the upper mantle may satisfactorily account for all the geochemical/petrological/
geophysical evidence that characterizes the Hoggar swell. Therefore the Hoggar volcanism, as well as other volcanic occurrences in
the Saharan belt, are likely to be related to passive asthenospheric mantle uprising and decompression melting linked to tensional
stresses in the lithosphere during Cenozoic reactivation and rifting of the Pan–African basement. This can be considered a far-field
foreland reaction of the Africa–Europe collisional system since the Eocene.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hoggar; mantle xenoliths; asthenospheric upwelling; lithosphere/asthenosphere interaction; lithosphere rejuvenation; metasomatism;
trace elements; Sr–Nd–Pb isotopes
⁎ Corresponding author.
E-mail address: [email protected] (L. Beccaluva).
0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2007.05.047
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
1. Introduction
The Cenozoic volcanism of the Hoggar region
(Algeria) represents one of the most important magmatic
district of the North African belt. It covers more than
10,000 km2 and is associated with a regional crustal
swell of Pan–African terranes, approximately 1000 km
in diameter (Fig. 1). The oldest Cenozoic volcanic rocks
are represented by a thick pile of tholeiitic plateau
basalts (35–30 Ma) that are characterized by an isotopic
signature typical of EM1-type enriched mantle (AïtHamou et al., 2000). Younger (Neogene–Quaternary)
volcanic rocks have a generally alkaline affinity and
include both basic lavas and trachyte–phonolite differentiates, with a prevalent HIMU (high μ) Pb–Nd–Sr
isotopic signature (Allègre et al., 1981; Azzouni-Sekkal
et al., 2007). In the Atakor–Manzaz districts, the latest
volcanic episodes are markedly alkaline (basanites and
nephelinites) sometimes entraining mantle xenoliths
which are the object of this study.
The Cenozoic volcanism appears to be linked to the
domal basement uplift of the Hoggar area, and could
reflect interaction between the lithosphere and the
underlying convecting mantle. This in turn may have
important implications for the existence of possible
mantle plumes and related geochemical components, as
well as for the role played by pre-existing tectonic
lineaments of the Pan–African lithosphere.
483
The mantle xenoliths from the Manzaz district
provide important constraints on the compositional
evolution of the lithospheric mantle beneath the area
and, by implication, on mantle sources from which the
Hoggar magmas were generated. Detailed studies of
mantle lithologies of the whole North African belt are
only reported for the In Teria district, NE Hoggar
(Dautria et al., 1992), and more recently for Morocco
(Raffone et al., 2001; Raffone et al., 2004) and Libya
(Beccaluva et al., in press). In this paper we present new
major and trace element analyses of both bulk-rocks and
constituent minerals, as well as Sr–Nd–Pb (He) isotopes
on separated minerals in order to define the geochemical
features of the main metasomatic components, their
provenance and role in the North African lithospheric
evolution. The results also contribute to a better understanding of the tectono-magmatic setting of the
Hoggar volcanism and its origin either from a deep
plume (Allègre et al., 1981; Aït-Hamou et al., 2000) or
to relatively shallow processes in the upper mantle
(Azzouni-Sekkal et al., 2007; Liégeois et al., 2005).
2. Petrography, mineral and bulk rock composition
Mantle xenoliths studied in this paper represent a
selection of nine representative samples (up to 8–9 cm
in size) from a larger xenolith population collected from
two scoria cones in the Manzaz volcanic district (N 23°
Fig. 1. Sketch map indicating locations of the main Cenozoic volcanic fields of North Africa, after (Liégeois et al., 2005) modified. Modal
compositions of the Manzaz mantle xenoliths in terms of olivine (ol), orthopyroxene (opx), clinopyroxene (cpx) are also reported.
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49.5′ 71ʺ–E 5° 53.7′ 20ʺ; N 23° 50.5′ 42ʺ–E 5° 50.5′
68ʺ). They are dominantly protogranular to equigranular
lherzolites and are characterized by a typical four phase
mineral assemblage. They show no evidence of basalt
infiltration.
The primary paragenesis in protogranular domains
consists of medium sized lobate olivine and orthopyroxene crystals (2–3 mm), and smaller interstitial green
clinopyroxene and brown spinel (isodiametric to holly
leaf in shape). The largest orthopyroxenes sometimes
show clinopyroxene exolution lamellae. Kink banding
is quite frequent in both olivine and orthopyroxene. The
evidence of strain and deformation in these crystals,
together with the absence of cumulate textures, rules out
a cumulate origin for these xenoliths.
Secondary metasomatic textures are scarce and
consist of rare spongy rims in clinopyroxene and
microcrystalline patches/lenses (particularly in sample
H58D) where glass is typically absent. These microdomains are widely obliterated by recrystallization
processes leading to equigranular textures, disappearance of unmixing lamellae and development of triple
junctions between the undeformed crystals (average
grain size less than 1 mm across).
Microprobe analyses showing compositional variations
even within individual crystals and in different textural
domains are reported in Table 1 in Appendix A of the
online supplement and analytical methods in Appendix B.
Olivine (ol) consists of rather homogeneous crystals
with Fo ranging between 89.1 and 91.3. Orthopyroxene
(opx) composition varies in the following ranges: En
87.2–89.7, Fs 8.8–10.7, Wo 1.2–2.1, with Mg # [Mg/
(Mg + Fe)] 0.89–0.91, showing an apparent equilibrium
with the coexisting olivine.
Clinopyroxene (cpx) has a wider compositional
range: En 46.4–54.6, Fs 4.5–7.1, Wo 38.4–47.7, with
Mg # 0.88–0.92, Cr # [Cr/(Cr + Al)] 0.05–0.13, Na2O
0.67–1.55 wt.% and TiO2 0.17–0.68 wt.%. As observed
in Fig. 2 the compositions of these clinopyroxenes
appear to be typical of fertile lherzolites (Mysen and
Kushiro, 1977; Jaques and Green, 1980), with the
relatively more depleted compositions (samples H58A,
H58B and H58D) partially overlapping the abyssal
peridotite field (Fig. 2a).
Clinopyroxene occurring as spongy rims and/or in reequilibrated equigranular domains, which are probably
related to metasomatic reactions, tends to have comparatively lower Na2O and CaO (and higher MgO) contents
(Fig. 2b).
Spinel (sp) shows the following compositional range:
Mg # 0.75–0.83, Cr # 0.09–0.22, conforming to the
compositions typical of fertile mantle peridotites, with
Fig. 2. Cr/(Cr + Al) vs. (Si + Mg)/Al (a) and Na vs. Mg (b) for
clinopyroxene from Manzaz mantle xenoliths as atoms per formula
units (a.f.u.) on the basis of 6 oxygens. In (a) compositional field of
abyssal peridotites (Johnson et al., 1990) also reported for comparison.
more refractory compositions (Cr # 0.20–0.22) observed in samples H58A, H58B and H58D.
Application of the two-pyroxene geothermometer
(Brey and Köhler, 1990) using the core composition of
equilibrated crystals gives nominal equilibration temperatures in the range of 950–1060 °C.
Major element mass balance calculations using
mineral phases and whole rock compositions (Table 2
in Appendix A of the online supplement; analytical
methods in Appendix B) provide the following modal
abundances: 64%–73% ol, 17%–22% opx, 8%–12%
cpx, 1–3% sp. As expected, lower cpx modal
abundances (8–9%) are recorded in three samples
(H58A, H58B and H58D) where the constituent phases
have more refractory compositions.
In the Al2O3 and CaO vs MgO diagrams (Fig. 3) the
Manzaz bulk-rock peridotites are compared with those
of other xenolith suites from Cenozoic volcanic
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
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Fig. 3. Bulk rock analyses of Manzaz lherzolite xenoliths in terms of Al2O3 vs MgO (a) and CaO vs MgO (b), compared to other mantle xenoliths
from the African plate (Gharyan, Libya (Beccaluva et al., in press); Iblean district, SE Sicily (Beccaluva et al., 2005)). The field of abyssal peridotites
(light grey) and residual trends after melt extraction (dark grey) from a fertile lherzolite source (PM) after Niu (1997) are also reported.
occurrences in the African plate (Gharyan district, Libya
(Beccaluva et al., in press); Iblean district, Sicily
(Beccaluva et al., 2005), as well as with abyssal
peridotites and theoretical mantle residua after progressive partial melting (Niu, 1997). Comparison with these
fields suggests that the Manzaz mantle xenoliths
underwent a moderate depletion related to previous
melting processes.
In the chondrite-normalized bulk-rock Rare Earth
Elements (REE) patterns of Fig. 4, Manzaz peridotites
have heavy (H)REE (Tb–Lu) that range between 1.1–
1.2 (H58A and H58B), to 1.8 (H58D), and 2.4 (H58C) ×
Fig. 4. Chondrite-normalized REE patterns for bulk rock lherzolite xenoliths from Manzaz. Normalizing factors after Sun and Mcdonough (1989).
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Fig. 5. Chondrite-normalized REE patterns for clinopyroxenes and orthopyroxene from Manzaz lherzolite xenoliths. Normalizing factors after
Sun and Mcdonough (1989).
chondrite. Since the HREE in spinel peridotites are
mainly hosted in clinopyroxene (Bedini and Bodinier,
1999), these patterns are inversely correlated with the
various degree of depletion observed in the different
samples. On the other hand, light (L) REE enrichment
with LaN/YbN up to 8.3, CeN/YbN up to 6.2 and NdN/
YbN up to 4 characterizes the most depleted samples
H58A, H58B and H58D suggesting a direct correlation
between mantle depletion and metasomatic enrichment.
This is commonly observed in mantle xenoliths worldwide and is experimentally attributed to the more
effective permeability of refractory (cpx-poor) peridotite
matrix to metasomatising agents (Toramaru and Fujii,
1986). Accordingly, in the least depleted xenoliths the
degree of metasomatic enrichment is less intensive and
generally restricted to the lightest REE (LaN/YbN b 3,
CeN/YbN b 1.3 and NdN/YbN b 1).
3. Pyroxene trace element composition
Representative trace element analyses of constituent
pyroxenes (opx and cpx) are reported in Table 3 in
Appendix A of the online supplement and analytical
methods in Appendix B. REE chondrite-normalized
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
patterns are shown in Fig. 5 and Primordial mantlenormalized incompatible element distributions are
available in the online supplement (Appendix C).
Coherently with the REE bulk rock data, the averaged
HREE distribution of clinopyroxene decreases from 10.4–
11.2 × chondrite at 10–12% of modal cpx to 6.9–
8.5 × chondrite at 8–9% of modal cpx. Excluding sample
H58G, which shows a severe LREE depletion, all samples
display variable LREE enrichment that is plausibly induced
by metasomatic processes. In samples containing 10–12%
modal cpx this enrichment is restricted to the lightest (more
incompatible) REE, i.e. La and Ce, thus displaying spoonshape patterns with LaN/YbN up to 3, CeN/YbN up to 1.6,
and NdN/YbN up 1.2. Samples characterized by lower
modal cpx (8–9%), such as H58A, H58B and H58D, show
the highest LREE enrichments with LaN/YbN up to 14.6,
CeN/YbN up to 11.9, and NdN/YbN up to 7.1.
The chondrite-normalized REE distribution in orthopyroxene (Fig. 5) is generally characterized by HREE
negative fractionation (TbN/YbN 0.1–0.4). LREE patterns are often upward concave with LaN/NdN varying
from 0.6 to 3 and generally mimic, at lower level, those
of the coexisting clinopyroxene.
In H58D, the REE patterns of clinopyroxene are
variable from nearly flat to LREE enriched. The lowest
LREE patterns, intermediate between those of clinopyroxene and orthopyroxene, have been observed in a cpxrich (equigranular textured) veinlet at the boundary with
orthopyroxene. This can be interpreted as evidence of
clinopyroxene neo-crystallization at the expense of
orthopyroxene by metasomatic reaction under conditions of high Ca activity.
487
Depletion and enrichment processes in mantle
parageneses can be successfully evaluated using
clinopyroxene REE patterns, since these processes are
effectively traced by HREE and LREE distributions,
respectively. Application of the model proposed by
Johnson et al. (1990) shows that, starting from a
common initial mantle source in the spinel stability field
(modal cpx: 20%; initial clinopyroxene, Cpx0: YbN = 14,
NdN/YbN = 1.03), fractional melting of ∼ 8% could
account for the H58H and H58G (modal cpx 11%)
clinopyroxene compositions (Fig. 6). Lherzolites H58C,
H58E, H58F, and H45 (modal cpx 10–12%) probably
suffered similar melting degrees but depart from partial
melting trends delineating a progressive NdN/YbN
increase due to slight metasomatic enrichment. The
remaining cpx compositions from the most depleted
lherzolites (H58A, H58B and H58D) require greater
degrees of partial melting (up to 15–20%) which is
consistent with their most depleted composition (modal
cpx 8–9%) and show the highest metasomatic enrichments (NdN/YbN up to 7). This confirms that the most
depleted cpx-poor peridotite domains were more
effectively infiltrated by metasomatising melts in
agreement with experimental evidence of melt connectivity in mantle rocks (Toramaru and Fujii, 1986).
Trace element modelling to constrain the metasomatising agent(s) has been performed using partition coefficients (Kd) for clinopyroxene/alkaline basic melts (Zack
and Brumm, 1998) and is illustrated in Fig. 7. Calculations have been carried out for the most homogenously
enriched clinopyroxene compositions (samples H58A,
H58B and H58D) where it can be assumed that mineral/
Fig. 6. (a) YbN vs. NdN/YbN diagram for clinopyroxene from Manzaz lherzolites. Depletion trends and melting degrees by batch and fractional
melting are calculated starting from Cpx0 initial source after Bianchini et al. (2007). Lherzolites with 10–12% of modal cpx may reflect less than 12%
melting with no or slight metasomatic enrichments; lherzolites with 8–9% of modal cpx (H58A, H58B, H58D) may correspond to higher melting
degrees and remarkable metasomatic enrichments. Chondrite normalizing factors are from Sun and Mcdonough (1989).
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Fig. 7. Primordial Mantle (PM)-normalized trace element patterns of the computed metasomatic agents that affected mantle xenoliths from the
Manzaz district. Calculations were performed on the metasomatised clinopyroxene compositions, using distribution coefficients cpx/alkaline melt
after Zack and Brumm (1998). Compositional envelopes of alkaline basic melts from the African plate (basanites, nephelinites, melilitites (AzzouniSekkal et al., 2007; Beccaluva et al., 1998; Bianchini et al., 1998; Janney et al., 2002)) and carbonatites (Nelson et al., 1998; Coltorti et al., 1993;
Smithies and Marsh, 1998) are reported for comparison. Normalizing factors after Sun and Mcdonough (1989).
melt equilibrium was approached. Comparison with
compositional fields of natural alkaline basic volcanics
and carbonatites reveals that the inferred metasomatic
agent(s) can be best equated with alkali-silicate melts such
as alkali-basalts, basanites and melilitites from the African
plate (Azzouni-Sekkal et al., 2007; Beccaluva et al., 1998;
Bianchini et al., 1998; Janney et al., 2002). However,
discrepancies are evident in the deficiency of Ti and
excess of Th for some theoretical compositions, thus
suggesting that the strongly alkaline metasomatic melts
could be also characterized by variable enrichment in
carbonatitic components, that in the extreme case could
reach a silico-carbonatitic composition. This seems to be
confirmed also by the LaN/YbN vs Ti/Eu diagram (Coltorti
et al., 1999) where the most enriched Manzaz cpxs plot
between the alkali silicate- and carbonatite-metasomatised clinopyroxene fields (figure in Appendix C of the
online supplement). The above conclusions agree with
those of Grégoire et al. (2000) and Delpech et al. (2004)
for Kerguelen mantle xenoliths that are interpreted to have
been affected by carbonate-rich silicate melts rather than
pure carbonatites. They also conform with findings
reported by (Dautria et al., 1992) for peridotite xenoliths
collected at In Teria (NE Hoggar margin) that emphasised
the alkali-silicate to carbonatite nature of the metasomatising agents.
4. Isotope characteristics of the Manzaz
mantle xenoliths
87
Cpx separates from the Manzaz xenoliths have
Sr/86Sr = 0.70214–0.70323, 143Nd/ 144 Nd = 0.51295–
0.51366, 206 Pb/ 204 Pb = 19.52–20.18, 207 Pb/ 204 Pb =
15.66–15.72, 208 Pb/204Pb = 38.62–39.93 (Table 4 in
Appendix A of the online supplement; analytical methods
in Appendix B). For both whole-rock and clinopyroxene,
Nd N /Yb N ratios are inversely correlated with
143
Nd/144Nd, with the higher isotopic ratios recorded in
samples containing 10–12% modal cpx (i.e. least affected
by metasomatism) and the lower values recorded in
samples with lowest modal cpx (8–9%) which appear
more effectively metasomatised. This confirms that the
permeability of the peridotite matrix, related to its clinopyroxene content (Toramaru and Fujii, 1986), also
influences the mantle isotopic fingerprint.
In the Sr vs. Nd isotope diagram (Fig. 8), the samples
with 10–12% modal cpx plot in the upper-left quadrant
close to the depleted mantle (DM) end-member, whereas
those with 8–9% modal cpx cluster around the HIMU
mantle end-member ( 87 Sr/ 86 Sr = 0.7031–0.7032,
143
Nd/144 Nd = 0.51292–0.51295). The HIMU Sr–Nd
isotope composition can be considered the predominant
signature of the metasomatic agents. The HIMU
fingerprint is also recorded in the Hoggar alkaline
lavas (Allègre et al., 1981) and, more in general, in all the
North Africa Cenozoic alkaline lavas (Jebel Marra
(Davidson and Wilson, 1989; Franz et al., 1999);
Gharyan district (Beccaluva et al., in press); Iblean and
Sicily Channel districts (Bianchini et al., 1998; Civetta
et al., 1998), and therefore it can be considered as a
ubiquitous component that affected the lithosphere on a
regional scale.
Lead isotopes, while unequivocally confirming the
HIMU signature of the metasomatising agent, also
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
489
Fig. 8. Sr–Nd–Pb isotopic variations for clinopyroxenes from Manzaz mantle xenoliths. Composition of mantle xenoliths from the Gharyan district
of Libya (Beccaluva et al., in press), and Cenozoic alkaline lavas from various African volcanic districts are reported for comparison: Hoggar (Allégre
et al., 1981); Jebel Marra (Davidson and Wilson, 1989; Franz et al., 1999); Iblean–Sicily Channel Districts (Beccaluva et al., 1998; Bianchini et al.,
1998; Civetta et al., 1998). Isotopic mantle end members (DM, HIMU, EMI and EMII) are reported after Zindler and Hart (1986).
display a clear decoupling with respect to Sr–Nd isotopes
in the least metasomatised samples. In fact, the enrichment in 206Pb (206Pb/204Pb = 19.5–20.1) and 208Pb
(208Pb/204Pb = 38.6–39.8) is significant even in samples
H58C, H58E, H58F, H58H that are considered largely
unaffected by metasomatism on the basis of 143Nd/144Nd
(0.51337–0.51366). This apparent incongruence between
the Sm–Nd and the U–Pb systems has also been observed
in the Gharyan mantle xenoliths (Beccaluva et al., in
press) and may be attributed to the greater mobility of Pb
compared to Nd (and Sr), as indicated by the respective
cpx/melt partition coefficients (Geochemical Earth Reference Model (GERM)) Staudigel et al., 1998. Therefore
Pb isotopes appear to be more sensitive indicators of
incipient metasomatic reactions.
The very high 143Nd/144Nd of H58E, H58F, H58H
(N 0.5135) suggests that the relative mantle domains were
affected by pre-Paleozoic partial melting events in the
presence of residual garnet resulting in high Sm/Nd that
ultimately led to the very high Nd isotopic ratios (Downes
et al., 2003; Bianchini et al., 2007). This implies that these
mantle rocks were subsequently re-equilibrated at a lower
depth in the current spinel stability facies. Significantly,
garnet-bearing lherzolites have been reported in mantle
xenolith population of In Teria volcanic district (NE
Hoggar margin (Dautria et al., 1992)).
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A helium isotope determination has been made on gas
from inclusions trapped in olivine from the metasomatised lherzolite H58B and gave 3He/4He of 6.6 Ra (where
Ra is the atmospheric composition 1.39 × 10− 6). Although the measured value is slightly lower than several
Hoggar lavas (Pik et al., 2006), similar ratios are
recorded in Cenozoic HIMU-related products from the
northern-central African lithosphere, e.g. peridotite
xenoliths from the Gharyan (Beccaluva et al., in press)
and Iblean volcanic districts (Sapienza et al., 2005), as
well as alkaline lavas from Etna (Marty et al., 1994),
Cameroon Line (Barfod et al., 1999) and Canaries
(Hilton et al., 2000). Indeed, they are consistent with the
rather uniform 3 He/ 4 He that characterizes HIMU
volcanism irrespective of the strength of Pb isotopes
and trace element ratio signal.
5. Discussion and conclusions
Most peridotite xenoliths from the Manzaz volcanic
district record metasomatic enrichment that overprinted
a previously depleted mantle lithosphere. Trace element
modelling of the metasomatised clinopyroxenes allows
us to infer that the metasomatising agents were highly
alkaline carbonated melts such as nephelinites/melilitites (or at their most extreme silico-carbonatites). They
are characterized by a clear HIMU Sr–Nd–Pb–He
isotopic signature.
In some cases there is a decoupling of Nd–Sr and Pb
isotopes. This may be explained by a greater mobility of
Pb (U, Th, La, Ce) that could be modelled by
chromatographic fractionation processes as proposed
by Bodinier et al. (2004). This decoupling is evident in
xenoliths containing more than 9% cpx, implying that
below this “petrological barrier”, a coherent metasomatic imprint for all isotopes is attained due to higher
permeability in the most refractory (cpx-poor) peridotite
domains (Toramaru and Fujii, 1986), where a sufficiently high metasomatic melt/matrix ratio occurs.
The Sr–Nd–Pb isotope composition typical of EM1type enriched mantle, that is observed for the older
tholeiitic plateau basalts of Hoggar (Aït-Hamou et al.,
2000), does not appear to be represented in the
lithospheric mantle sampled by the xenoliths. Since the
EM1 signature can originate as an old component of the
Pan–African lithosphere (Cottin et al., 1998; Aït-Hamou
et al., 2000), its absence in mantle xenoliths (and the
associated alkaline lavas) is perplexing. This cannot be
attributed to a deeper provenance of mantle xenoliths
with respect to the EM1 mantle portions where tholeiitic
basalts were originated, owing to their comparable depth
range of origin (1–2 GPa (Green and Fallon, 2005)).
More convincing explanations involve the lateral
displacement of an old lithospheric mantle by upwelling
asthenosphere from the underlying convective mantle. The
resulting “rejuvenated” lithosphere may represent a single
mantle column from which both basic alkaline melts could
have been produced at deeper depths, and mantle xenoliths
have been sampled at shallower levels. This is supported
by the interpretation of the mantle xenoliths as garnet (gt)bearing mantle domains re-equilibrated to spinel lherzolites in accordance with the high 143Nd/144Nd observed in
several samples. Garnet (and spinel) peridotite xenoliths
have been found in the In Teria volcanic district along the
NE Hoggar margin (Dautria et al., 1992), which plausibly
represents the older cratonic lithospheric mantle. Significantly, the In Teria spinel peridotite xenoliths record
widespread pyrometamorphic textures (secondary aggregates of olivine, clinopyroxene, spinel and feldspar
essentially formed from primary orthopyroxene and spinel
(Dautria et al., 1992)) that testify to reactions with
metasomatic melts at shallow mantle depths, probably in
the more rigid mechanical boundary layer. Similar reaction
textures are also recorded in mantle xenoliths from other
volcanic districts of the North African belt (Iblei, Sicily
(Beccaluva et al., 2005); Gharyan, Libya (Beccaluva et al.,
in press)) but have not been observed in the Manzaz
xenoliths in which metasomatism seems to be texturally
re-equilibrated. This probably means that metasomatic
reactions beneath the central Hoggar occurred at comparatively greater depth, i.e. in the thermal boundary layer,
prior and/or during mantle uprising. It is conceivable that
at these depths, higher temperatures and more effective
elemental diffusion could have facilitated re-equilibration
processes preventing the formation/persistence of pyrometamorphic textures.
The Manzaz spinel–lherzolites could be interpreted
as representative of an upwelling asthenospheric mantle
diapir that accreted to the older cratonic lithosphere.
Relics of the cratonic lithosphere are probably represented by the variably metasomatised garnet- to spinel–
peridotite xenoliths recovered at In Teria (Fig. 9). These
mantle dynamics certainly contributed to lithosphere
bulging that resulted in the Hoggar swell, and are
consistent with the geophysical evidence such as the
gravity field data (− 90 mgal (Lesquer et al., 1988)) and
the thermal anomaly centred on the Atakor volcanism
(heat flow 63 mW/m2 (Lesquer et al., 1989)). Recent
fission track age data suggest that the regional uplift of
the Hoggar basement started at least as early as the
Cretaceous (Khaldi et al., 2006). Seismic tomography of
the North African plate confirms a variable lithospheric
thickness in the Saharan area due to local asthenospheric
upwelling (Ayadi et al., 2000), but seems to rule out the
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
491
Fig. 9. Simplified cartoon showing the hypothesised lithosphere/asthenosphere interaction beneath the Hoggar swell. (a) Tholeiitic plateau basalts
(35–30 Ma, Anahef district) formed by high melting degrees of EM1-metasomatised mantle of the Pan–African cratonic lithosphere. (b) Younger
alkaline volcanism of the Atakor–Manzaz district generated from low melting degrees of HIMU-metasomatised mantle sources. The latter probably
represent a rejuvenated lithosphere formed by intra-cratonic upwelling and accretion of asthenospheric material. The Manzaz spinel lherzolite
xenoliths may therefore represent fragments of this rejuvenated lithosphere, whereas the garnet/spinel peridotite xenoliths from In Teria may represent
the older cratonic mantle of the Pan–African lithosphere.
existence of deep mantle plumes (Davaille et al., 2005;
Sebai et al., 2006).
The systematic occurrence of Neogene alkaline lavas
and associated mantle xenoliths with a clear HIMU
affinity across the African plate indicates that it is a
ubiquitous sub-lithospheric component across CentralNorthern Africa. The widely accepted genetic interpretation of the HIMU end-member may fit with the
framework delineated above. In fact this component is
considered related to the ultimate fate of subducted
oceanic lithotypes which create high U/Pb and Th/Pb
mantle domains that, after a long-term storage, result in
highly radiogenic Pb compositions (Weaver, 1991;
Carlson, 1995; Hofmann, 1997). Accordingly, Wilson
and Patterson (2001) physically locate this component
above the 670 km discontinuity in the upper mantle,
where geophysical evidence shows subducted slab relics
flattened over wide upper mantle regions (e.g. in the
Central Mediterranean (Faccenna et al., 2001)).
Helium isotopes carried out on Cenozoic volcanics
and associated mantle xenoliths across Central-Northern
Africa (Ra b 9; (Beccaluva et al., in press; Pik et al.,
2006)) also corroborate the interpretation of HIMU as a
common regional sub-lithospheric component. The
relatively low 3He/4He ratios observed for the Saharan
districts represent a further indication that this metasomatic component is confined in the upper mantle, unlike
the Ethiopian–Yemen plateau basalts (3He/4He up to 20
Ra) which have been related to a deep mantle plume
possibly generated from the core–mantle boundary (Pik
et al., 2006)). By contrast, relatively low 3He/4He can be
satisfactorily explained by degassing of shallow mantle
domains and/or the addition of recycled components to
the upper mantle (Moreira and Kurz, 2001).
In conclusion, convection within the upper mantle
and lithosphere/asthenosphere interactions taking place
mainly along lithospheric discontinuities such as craton/
mobile belt borders, as proposed by the edge-driven
convection model (King and Anderson, 1995; King and
Anderson, 1998), can be considered the most appropriate
geodynamic scenario for the mantle evolution of the
Hoggar swell. Whatever the complex terminology of the
492
L. Beccaluva et al. / Earth and Planetary Science Letters 260 (2007) 482–494
convective instabilities in the mantle, referred to various
types of plumes and hot-spots (Courtillot et al., 2003),
the geochemistry of the Hoggar xenoliths seems to be
satisfactorily accounted for by shallow convective
processes with the origin of the related geochemical
components confined within the upper mantle. Therefore, the Hoggar volcanism, as well as other volcanic
occurrences of the Saharan belt, are likely to be related to
passive asthenospheric mantle uprising and its decompression melting linked to tensional stresses in the
lithosphere during tectonic reactivation and rifting of the
Pan–African basement. This can be considered a farfield foreland reaction of the Africa–Europe collisional
system operational since the Eocene (Azzouni-Sekkal
et al., 2007; Liégeois et al., 2005).
Acknowledgements
The authors gratefully acknowledge the constructive
criticism of M. Grégoire and of an anonymous referee
that greatly improved the manuscript. Particular thanks
are extended to R. Carlson for his suggestions and
encouragement. The SUERC laboratories are supported
by the Scottish Universities.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
epsl.2007.05.047.
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